Technical Abstract:
Competitive exclusion (CE), the inoculation of young animals with healthy gut microbes, has been shown to enhance colonization resistance of newborn piglets to Salmonella Choleraesuis (SC). Since reduced shedding of these pathogens is observed, we hypothesized that this should reduce the potential for their horizontal transmission. To test this hypothesis, we orally challenged pigs at 15 days of age (1 day post weaning) with 10E8 colony forming units (CFU) of a novobiocin and nalodixic acid resistant strain of SC. These experimentally infected pigs (designated as seeders) were then commingled 1 to 2 days later with an equal number of pigs having no known prior Salmonella exposure (designated as contacts). Our experimental design consisted of: 1) commingling CE-treated seeders with CE-treated contacts, 2) commingling untreated seeders with untreated contacts, 3) commingling CE-treated seeders with untreated contacts and 4) commingling untreated seeders with CE-treated contacts. Competitive exclusion treatment was given at birth. All seeders (whether CE-treated or not) were infected by SC at 23 to 26 days of age, as determined by culture of ileocolic lymph nodes and cecal contents collected by necropsy. The incidence of shedding of SC, as determined by daily culture of rectal swabs, was less for the CE-treated seeders (42%) than for the untreated seeders (49%). Horizontal transmission occurred readily between untreated seeders commingled with untreated contacts, with 10 of 13 contacts becoming infected. In contrast, only 3 of 13 CE-treated contacts became infected when commingled with CE treated seeders thus demonstrating a benefit of CE treatment in reducing the spread of SC. As expected, CE worked best if given to both seeders and contacts.